Mem: Fix issue with dirty block being losted when entire block transfered to non-cache.

This change fixes the problem for all the cases we actively use. If you want to try
more creative I/O device attachments (E.g. sharing an L2), this won't work. You
would need another level of caching between the I/O device and the cache
(which you actually need anyway with our current code to make sure writes
propigate). This is required so that you can mark the cache inbetween as
top level and it won't try to send ownership of a block to the I/O device.
Asserts have been added that should catch any issues.